The term “skin target”, is used to refer to an unwanted skin defect such as vascular and pigmented lesions, unwanted hairs, and acne. Selective thermolysis of a skin target refers to heating a skin target to a temperature sufficiently high to destroy the target (about 60–70° C.) without heating the surrounding healthy skin to a damaging level. In this method, the skin is irradiated with laser or incoherent light having a wavelength allowing it to penetrate into the tissue to the depth of the target. This method, however, works only when the target has a higher optical absorption coefficient that the surrounding tissue (i.e. the target is darker than the surrounding skin), otherwise it is not possible to sufficiently heat the target without heating the surrounding tissue to a damaging temperature. For example, a dark hair can be successfully destroyed in light skin. However, treatment of gray hairs is not effective due to a lack of melanin in the hair shaft to absorb the optical energy. Similarly, use of optical energy to destroy a skin target can only be applied with limited efficiency with dark skin. In order to reach the target such as a hair root or blood vessel, the light has to pass through the epidermis. A dark epidermis absorbs more light energy than a light epidermis. Thus, a higher light intensity must be used with dark skin, which increases the risk of burning the skin.
U.S. Pat. No. 5,755,753 describes a method for selective removal of hair using laser radiation, where the parameters of irradiation are optimized for selective light absorption by the melanin of the hair shaft.
An alternative method of tissue heating used in medicine is radio frequency (RF) energy. The electrical current generates an amount of heat in the tissue that depends on the current density and tissue conductivity. DC or AC electrical current causes muscle spasms and is dangerous for treatment, but high frequency RF current (>300 kHz) causes a pure thermal effect and can be used for skin treatment. The range of the frequencies higher than 20 MHz is difficult to implement. However, RF current is used only for non-selective treatment, i.e. it does not preferentially heat a skin target without overheating the surrounding skin. Therefore, the main applications of RF current are electro-surgery and blood coagulation. In both of these applications, the tissue is heated in an area adjacent to the applied electrodes to a coagulation or ablative level. For example, U.S. Pat. No. 5,122,137 describes a coagulator with controlled temperature. The U.S. Pat. No. 5,755,753 describes a device for non-selective heating of the dermis and epidermis with a unipolar RF system in combination with cooling of the epidermis. Using this method, it is possible to obtain a temperature of the dermis that is higher than the temperature of the epidermis. This method however does not provide selective heating of the target by the RF current and is therefore limited to only one application.